I have read of people using wooden dowels, so I think your idea would work. I would cut threads on each end so you could put rubber washers and nuts on to keep water from coming in. Personally, I don't lock my keel down. I like the ability to raise or lower quickly, depending on my point of sail or close proximity to shallow water. I have small expanding rubber plugs in the holes to keep water out. I do have a lockdown bolt, with rubber washers and a wingnut, stored right by the hole in case I would feel the need to put it in.